It has been suggested that electronic component designs in automotive vehicles can be simplified as to complexity and weight by assembling the components on thermoplastic substrates, e.g., on the underside of vehicle dashboards. This type of system has been termed Instrument Panel (IP) Super-Integration. In this system, individual electronic circuits are first assembled on thermoplastic substrates, with electronic components attached. Bus connectors, made of copper, extend beyond each such individually assembled circuit, and these bus connectors are to be joined so as to serve as interconnects between different circuits. For this purpose, materials are needed to metallurgically join the bus connectors at low temperatures, so as not to inflict thermally induced damages on the components, thermoplastic substrates with low heat deflection temperatures (HDT), and more importantly, the solder interconnects that have already been formed on each individual circuit.
Solders provide inexpensive, mass-producible and generally reliable interconnections to complete electronic circuitry between various elements that make up an electronic assembly. The solder joints, in addition to being electrical interconnects, serve as the mechanical attachment of the electronic components to the printed circuit board and also to transfer heat. Soldering makes these connections at temperatures below those that might cause damage to some of the elements of the assembly and substrate materials. Eutectic tin/lead (63Sn/37Pb) solder (melting temperature 183.degree. C.) is commonly used for reflow soldering with peak reflow temperature of 210-220.degree. C. so as to completely melt the metals in the solder. In order to use low cost thermoplastic substrate materials and some components which may be temperature-sensitive for integration, it is required to reflow at lower peak temperatures than 200.degree. C.
In such integration applications, the solder joints may experience significant thermal cycling during use. It is known that eutectic 63Sn/37Pb solder joints are reliable during thermal cycling between -40.degree. C. and 125.degree. C. Solders with melting temperatures lower than 180.degree. C. may not be able to withstand this thermal cycling over extended periods of time as is often encountered in automotive use. Many of the low melting temperature solder compositions also have a wide pasty range (up to 30.degree. C.) between solidus and liquidus temperatures that may allow damage to the solder joints during reflow soldering/cooling processes.
Metallic adhesives are an alternative to conventional metal alloy solders and generally comprise a mixture of a liquid metal and particulates. The adhesives form the solder joint when metal (or metal alloy) powders in the metallic adhesives react with the liquid metal (or metal alloy). The formed solder joint has a high post-curing melting temperatures, thereby providing a reliable solder interconnect for automotive use as is the intent of the present invention. Thus metallic adhesives capable of joining electrical connectors like bus connectors at low temperatures might be well adapted for use in , e.g., super-integration systems.
Metallic adhesives have been suggested in several patents. For example, U.S. Pat. Nos. 5,328,087 and CIP 5,445,308 (Nelson et al.) disclose mixing a thermally conductive filler containing a liquid metal into an unhardened adhesive material like epoxies or silicone gels. Disclosed liquid metal is, e.g., gallium, gallium/indium or gallium/tin combinations. The filler can be a gallium/copper/nickel paste which can solidify by amalgamation. One drawback in using these materials to join electrical connectors is the limited metallic contact between the adhesive and the electrical connectors to be jointed. It is appreciated that a continuous metallic medium is critical for reliable electrical connection. Melton et al. in U.S. Pat. No. 5,429,292 disclose a solder paste of compositionally distinct metal powders where a first metal powder of tin/bismuth solder alloy is combined with a second metal powder containing gold or silver. One disadvantage of this solder is that is uses a very expensive metal, gold, and it therefore is unsuitable for large volume use. In another U.S. Pat. No., 5,221,038, Melton et al. disclose a solder connection formed based upon a tin/indium or tin/bismuth alloy, having a melting temperature greater than the melting temperature of the initial solder alloy.
However, the post-soldering temperature is believed to be still too low for the automotive applications intended for the present invention. In U.S. Pat. No. 5,170,930, Dolbear et al. disclose a liquid metal paste which remains non-solidified in order to make detachable connections. The paste comprises an equilibrium mixture of a liquid metal like gallium or mercury and particulate solid like alumina, cobalt, or glass beads. Obviously, such a material has a specialized use for making detachable connections and is not intended for making a solid and durable electrical interconnection.
It would be desirable to have a metallic adhesive which overcomes the disadvantages of prior art solders and metallic adhesives. One object of such an adhesive would be to be able to join electronic components, e.g., electronic bus connectors, at low temperatures. Such a material would be particularly suitable for use in super-integration systems as are envisioned in the future. Yet another object of the present invention is to have a metallic adhesive that is free of toxic ingredients such as mercury, cadmium, lead, etc. The invention described herein provides metallic adhesives having these properties and excellent electrical and thermal conductivity.